Among thermal printers, a dye thermal transfer printer capable of printing a clear full color image is recently attracting particular attention. In a dye thermal transfer printer, a dye layer containing a dye of an ink ribbon is superposed on an image receiving layer of a receiving sheet, and the dye of the dye layer in a required portion is transferred at a predetermined concentration onto the receiving layer by the effect of heat supplied from a thermal head or the like, whereby an image is formed. The ink ribbon comprises dye layers for three colors of yellow, magenta and cyan or dye layers for four colors additionally including black. A full color image is obtained by repeatedly transferring respective color dyes of the ink ribbon in sequence to a receiving sheet.
With the progress of a digital image processing technique using a computer, the image quality or the like of a recorded image is remarkably enhanced and the market for thermal transfer system is expanding, but there is a demand for image quality and glossy texture comparable to those of a silver salt photograph. Also, as the technique of controlling the temperature of a thermal head is improved, the demand for a high-speed high-sensitivity printing system is increasing. To cope with such requirements, how efficiently the heat value of a heating device such as thermal head is utilized for the image formation becomes an important problem to be solved.
A receiving sheet generally comprises a support and a receiving layer formed on the surface thereof. When a normal film is used as a substrate for the support, despite excellent smoothness, the heat from a thermal head may escape to the substrate to give rise to insufficient recording sensitivity, or since a film is lacking in the satisfactory cushioning property, the ink ribbon and the receiving sheet may fail in closely contacting with each other and this may cause density unevenness or the like.
In order to solve these problems, there have been proposed supports, for example, a support obtained by laminating a foamed film on a core material layer such as paper sheets (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 61-197282 (page 1)), and a support obtained by laminating a biaxially stretched film (synthetic paper) mainly comprising a thermoplastic resin such as polyolefin resin and containing a void structure, on a core material layer such as paper sheets (see, for example, Kokai No. 62-198497 (page 1)). The receiving sheet using such a support is excellent in the heat insulating property and smoothness but, disadvantageously, the receiving sheet is dimpled due to heat and pressure at the transportation or printing in a printer and the appearance is impaired.
Furthermore, the foamed film is expensive or a thick foamed film needs to be used in order to control the thickness of the entire receiving sheet to a desired thickness, which incurs a problem that the profitability is low or a problem that the texture of the obtained receiving sheet differs from that of a silver salt photographic printing paper.
When a paper sheet is used as the support substrate of the receiving sheet, the heat from a thermal head disadvantageously escapes to the substrate to render the recording sensitivity insufficient. The cushioning property of paper sheets is somewhat higher than that of a film, but the close contact between the ink ribbon and the receiving layer becomes non-uniform due to uneven fiber density of paper and the print comes to have irregular shading.
In order to solve these problems, a receiving sheet where an intermediate layer containing hollow particles is provided between a paper support and a receiving layer has been disclosed (see, for example, Kokai Nos. 63-87286 (pages 1 and 2) and 1-27996 (pages 1 to 3)). In this receiving sheet, the hollow particle-containing layer provides an effect of enhancing the heat insulating property or cushioning property to thereby improve the sensitivity or image quality, but there arises a phenomenon that releasability between the receiving layer and the ink ribbon at the printing is poor as compared with the case of using a support or the like obtained by laminating a foamed film on a core material layer such as paper sheets. In other words, fusion-bonding is liable to occur.
This is considered to arise because of the following reason. A polyisocyanate is generally blended in the receiving layer for the purpose of three-dimensionally crosslinking a release agent or a thermoplastic resin so as to prevent fusion-bonding with the dye layer of an ink ribbon (see, for example, Kokai No. 10-129128 (pages 2 to 4)), but since the moisture contained in paper sheets selectively reacts with the polyisocyanate, desired three-dimensional crosslinking cannot be achieved for the resin of the receiving layer and this leads to a failure in obtaining a sufficiently high effect of preventing fusion-bonding. In this respect, an improvement is demanded.
Also, the moisture content of the receiving sheet after allowing the receiving sheet to stand for one day in a fixed temperature/humidity atmosphere is specified and this is considered to have reached almost equilibrium, but the moisture content during or immediately after the production is not known. Furthermore, for example, formation of a waterproof layer between a paper substrate and a foamed layer, or formation of an anticurling layer on the back surface side of a substrate has been disclosed (see, for example, Kokai No. 8-25811 (pages 2 to 4)). However, the fusion-bonding between the receiving layer and the ink ribbon at printing as referred to in the present invention is mainly attributed to the performance of the receiving layer, and the performance of the receiving layer is considered to be greatly affected by the receiving layer components such as crosslinking agent or by the construction of hollow particle-containing intermediate layer, barrier layer or the like in the vicinity of the receiving layer.
As for the adhesive resin used in the intermediate layer, it has been proposed, for example, to use an organic solvent-resistant resin (preferably polyvinyl alcohol, casein, starch or the like) (see, for example, Kokai No. 1-27996 (pages 1 to 3)) or a resin having a minimum film-forming temperature of 25° C. or more (see, for example, Kokai No. 7-17149 (page 2)). However, when such a resin is used alone, there arises a problem that uniform formation of the intermediate layer or formation of a flexible layer becomes difficult. In this respect, an improvement is demanded.
Also, a void distribution in the surface coating layer of a transfer sheet as measured by a mercury press-fitting porosimeter (see, for example, Kokai No. 7-98510 (page 2)), a dynamic hardness on the surface of a thermal transfer ink-receiving layer (see, for example, Kokai No. 2002-11969 (page 2)), and the like have been disclosed, but such properties are used involved in a fusion-type thermal transfer system or an electrophotographic system and are limited to the characteristics of the receiving layer surface.
A receiving sheet using a paper substrate as the support is relatively inexpensive and can form an image with a sufficiently high density by providing an intermediate layer, but this receiving sheet is disadvantageously liable to absorb environmental moisture and readily brings about warpage, so-called curling, due to fluctuation of humidity. Furthermore, although a coating layer such as intermediate layer and receiving layer is provided on one surface of the receiving sheet, such a coating layer generally has very small moisture absorption as compared with paper and the difference in the degree of moisture absorption from the paper substrate gives rise to generation of curling. More specifically, so-called top curling is generated on the receiving layer surface side in a high-humidity environment because the paper support tends to absorb moisture and expand, whereas so-called back curling is generated on the side opposite the receiving layer in a low-humidity environment because the paper substrate tends to shrink.
For various purposes such as improvement of printing/traveling performance, a backside layer is provided on the back surface (surface opposite the intermediate layer or receiving layer) of the receiving sheet. For example, with respect to the resin for the formation of the backside layer, a method of using a polyvinyl acetal resin and an acryl resin having a glass transition point of 50° C. or more in combination has been disclosed (see, for example, Kokai No. 4-161383 (page 1). However, this backside layer is intended mainly to, for example, improve non-dyeability or prevent electrostatic charge, and the anticurling property is not necessarily satisfied. In order to render the backside layer effective for the curling correction, a resin having good film-forming property needs to be coated to form a highly elastic film.
Also, as high-speed high-sensitivity processing of a thermal transfer recording system proceeds, the heating value supplied at printing from a thermal head to a receiving sheet is increased and at the same time, a back printing failure tends to readily occur. The back printing failure is a problem such that when the front and back of a receiving sheet are mixed up at the loading of receiving sheets into a thermal transfer printer and printing is performed, the ink ribbon and the back surface of a receiving sheet are fusion-bonded and paper jamming is caused. The back surface of a receiving sheet is demanded to possess a fusion-preventing property so as to allow for paper discharging without fusion-bonding of the ink ribbon and the backside layer even at back printing.
It is known to add various fillers for imparting back printing suitability to the backside layer of a receiving layer. By the addition of a filler, the backside layer can be made slippery and the ink ribbon can be prevented from fusion-bonding with the back surface of a receiving sheet due to heat of a thermal head at back printing. As for the filler, organic or inorganic fine powders, fine particles or fine particle emulsions and the like have been proposed.
For example, for the purpose of ensuring printing/traveling performance, antiscratching or the like, a method of using a resin and a filler of the same species as the resin for the backside layer and causing the filler to be not exposed but covered with the resin (see, for example, Kokai No. 8-25814 (page 2)), or a method of incorporating an organic filler having a particle diameter of 0.5 to 30 μm into the backside layer and adjusting the surface roughness to from 0.3 to 3.0 μm (see, for example, Kokai No. 9-123623 (page 2)) have been proposed. However, means for preventing curling in a high humidity environment, which is peculiar to a paper support, is not disclosed.
Also, a method of incorporating spherical particles having an average particle diameter of 2 to 6 μm and an average particle diameter of 8 to 15 μm into the backside layer (see, for example, Kokai No. 7-137464 (page 4)) has been proposed. However, as indicated in its Examples, polyvinyl alcohols in general have a property of absorbing moisture in a high humidity environment and thus this method has a drawback that in the case of a normal paper support, the curl-preventing effect extremely decreases. Furthermore, a method of using a polyvinyl acetal resin, a polyacrylic acid ester resin and a particle having Mohs hardness of 1 to 4 for the backside layer has been proposed (see, for example, Kokai No. 6-239036 (page 2)), but this method is disadvantageous in that the hardness as the filler is too high and when receiving sheets are superposed one on another, the receiving layer in contact with the backside layer is scratched by the filler and thus the output image is deteriorated.
With respect to the method for enhancing the anticurling performance, a method of using an acryl polyol resin and a filler for the backside layer has been proposed (see, for example, Kokai No. 8-118822 (page 2)), but a polyester film is used as the support and water resistance of the acryl polyol itself is disadvantageously not sufficient. Also, a method of providing a water-vapor barrier layer such as vinylidene chloride resin on the back surface of a paper substrate has been disclosed (see, for example, Kokai No. 11-34516 (page 2)), but a chlorine-based resin has a problem in view of environmental consideration.